This invention relates to a semiconductor device, a method of making the same, a circuit board, and a flexible substrate, and, in particular, to a semiconductor device, a method of making the same, a circuit board, and a flexible substrate in which a flexible substrate is disposed above an active surface of a semiconductor chip.
If high-density mounting of semiconductor devices is pursued, bare-chip mounting would be ideal. However, quality assurance and handling are difficult in the bare chip state. To that end, a semiconductor device has been proposed in which a bare chip is packaged to form a package of a size that is close to the size of the bare chip, as disclosed in, for example, International Publication WO95/08856.
This semiconductor device is made as described below. A flexible substrate (abbreviated to substrate) is disposed above the active surface of a semiconductor chip. External electrodes for mounting are provided on this flexible substrate. Leads provided on the flexible substrate are then connected to electrodes of the semiconductor chip while being cut, and a resin in gel form is injected between the semiconductor chip and the flexible substrate to complete the semiconductor device.
With this semiconductor device, inspections can be done reliably in the packaged state, it is possible to ensure product quality because the resin between the semiconductor chip and the flexible substrate covers the active surface of the semiconductor chip, and handling is also easy.
However, with this technique, the leads must be cut one by one and bonded one by one (single-point bonding). If an attempt is made to cut and connect all of the leads at the same time, the support of the flexible substrate disappears and therefore the connection positions of the leads and the electrodes are placed at different positions. This means that the technique disclosed in the above Publication cannot be applied to a batch connection method for all of the leads. This is inferior to batch connections (gang bonding) from the mass production viewpoint.
In addition, since the substrate itself is flexible, various problems caused by flection of the substrate cannot be solved. For example, when the resin in gel form is injected between the semiconductor chip and the substrate, there is a strong possibility that the injection will be uneven, because of this flection. In addition, the external electrodes are positioned on the top of the flexible substrate, so that it is not possible to fix the positions thereof absolutely, and difficulties are likely to occur, particularly during the connection to the external substrate.
In addition, a flexible substrate has to be held by a special jig positioned so as to surround the semiconductor chip, and that jig must be prepared anew.
The present invention was devised in order to solve the above described problems and its objective is to prepare a semiconductor device, a method of making the same, a circuit board, and a flexible substrate that are easy to assure the quality and easy to handle, and are extremely reliable during fabrication.
This invention further provides a semiconductor device, a method of making the same, a circuit board, and a flexible substrate that have superlative mass production capabilities and enable the use of existing fabrication devices without modification in the fabrication thereof.
A method of making a semiconductor device in accordance with this invention comprises:
a step of preparing a flexible substrate that has a region overlapping a semiconductor chip, the flexible substrate having external electrode formation portions where external electrodes are formed, the external electrode formation portions are formed within the overlapped region;
a step of providing a gap preservation member on at least one of a surface having electrodes of the semiconductor chip and a surface of the flexible substrate that is disposed facing the surface having electrodes of the semiconductor chip; and
a step of arranging the semiconductor chip and the flexible substrate with surfaces thereof facing one another, in a state in which the gap preservation member is interposed therebetween, and connecting connection portions formed on the flexible substrate to the electrodes of the semiconductor chip.
With this invention, the preparation of the above-described flexible substrate makes it possible to provide a package that is of the same size as the chip. In this state, a surface of a semiconductor chip having electrodes is arranged so as to face a surface of a flexible substrate that is disposed facing that surface of the semiconductor chip having electrodes, in other words, a surface of the flexible substrate on a side on which connection portions to electrodes are positioned. A gap preservation member is provided on at least one of these surfaces. Since the semiconductor chip and the flexible substrate are arranged with the gap preservation member therebetween, a gap can be guaranteed reliably between the two components. This makes it unnecessary to provide a jig for preserving this gap. Since this constant gap is held between the two components from the step of assembling the semiconductor chip and the flexible substrate onward, unexpected electrical short between the two components can be prevented. In addition, the connection portions of the flexible substrate and the electrodes of the semiconductor chip are connected together in a state in which the gap preservation member is interposed therebetween, so that the gap preservation member acts as a support shaft during the connection, enabling reliable connection.
The gap preservation member formed during the step of providing the gap preservation member is preferably provided within a region that excludes a region corresponding to the external electrode formation portion. In other words, the gap preservation member is not provided in a part corresponding to the external electrode formation portion for the formation of external electrodes. This ensures that the external electrodes can move readily because they are not fixed by the gap preservation member, thus facilitating the relief of thermal stresses.
The batch connection of the connection portions and the electrodes during this connection step is also preferable from the viewpoint of mass production. Note that the problem of flection of the flexible substrate is most likely to occur during the batch connection, but since the connection is performed in a state in which the flexible substrate is held by the gap preservation member, this makes it possible to prevent the problem of flection of the flexible substrate.
In addition, the method could further comprise a step of forming a stress absorption layer between the semiconductor chip and the flexible substrate.
This stress absorption layer absorbs thermal stresses caused by the difference in coefficients of thermal expansion between the semiconductor chip and the flexible substrate, and also thermal stresses caused by the difference in coefficients of thermal expansion between the semiconductor chip and an external connection substrate (a mounting substrate). In addition, if the stress absorption layer is formed in the state in which the gap preservation member is interposed, the stress absorption layer can be formed in a state in which the gap is reliably held, and thus the stress absorption layer can be formed easily and also reliably.
Stress relief is particularly effective if the stress absorption layer is provided in at least a region corresponding to the external electrode formation portion.
The gap preservation member could be provided by printing a resin. For example, this gap preservation member could be formed by using a screen-printing method to print a solder resist. If a printing method is used, an existing printing device could be adapted therefor, which is advantageous because it makes it possible to reduce the costs of fabrication.
Alternatively, the gap preservation member could be provided by the ejection of a resin by an ink-jet method. The ink-let method in such a case is a method that is widely used in printers, for printing marks on defective products after semiconductor chips have been inspected, for example. In this method, a nozzle is used, and fine particles of resin ejected from that nozzle are blown towards the object to be printed. Note that a resin that does not clog the head should be used with this ink-jet method. If such an ink-jet method is used, many of the preparation steps that are required before printing (such as setting the ink, squeegee, or plate) become unnecessary, enabling a further shortening of the process than the printing method. Since the gap preservation member can be provided with no mechanical contact with the active surface, even if it is formed on the active surface side of the semiconductor chip, for example, this is preferable from the viewpoint of security of the active surface of the semiconductor chip.
When an ink-jet method is used, it is particularly preferable that the gap preservation member is provided only on the surface of the semiconductor chip that has the electrodes. Positional accuracy between the head and the object to be printed is usually required with an ink-jet method. A semiconductor chip is particularly useful as regards positional accuracy, because it has a rigid substrate. In addition, if an ink-jet method is employed, it is possible to use a device that puts bad marks on a surface to indicate a defective semiconductor chip, thus enabling the adaptation of existing equipment therefor.
The stress absorption layer could be formed by injecting a molding material. Injection of a molding material makes it possible to form the stress absorption layer in a region that is enclosed by the overlapping semiconductor chip and flexible substrate. In addition, the resin can be spread reliably between the semiconductor chip and the flexible substrate, ensuring that there are no voids between the surface of the semiconductor chip and the flexible substrate, preventing the accumulation of moisture, and also preventing corrosion.
A thermosetting resin could be used as this resin. Alternatively, an ultraviolet-setting resin could be used as this resin.
A material having properties of absorbing stresses between the semiconductor chip and the flexible substrate could be used as the gap preservation member, and the material also forms a stress absorption layer by hermetically sealing the opposing surfaces of the semiconductor chip and the flexible substrate.
In such a case, the gap preservation member is also formed as a stress absorption layer by hermetically and reliably sealing the two opposing surfaces of the semiconductor chip and the flexible substrate.
This configuration makes it possible to shorten the process, because the gap preservation member also functions as a stress absorption layer.
Since molding materials are expensive, making their use unnecessary leads to a reduction in costs.
The gap preservation member could also have thermoplastic properties, and the step of forming a stress absorption layer could comprise a step of applying heat and pressure to the gap preservation member. Since this gap preservation member has thermoplastic properties, the application of heat thereto forms a stress absorption layer. The gap preservation member also makes it easy for the completed semiconductor device to absorb stresses when heat is applied thereto.
During the step of applying pressure, it is preferable that the position at which pressure is applied to the gap preservation member is gradually shifted, to perform a sequence of local pressure applications.
This gap preservation member is preferably provided only on a surface of the flexible substrate disposed towards the surface of the semiconductor chip that has electrodes.
The step of providing the gap preservation member preferably comprises a step of forming a wiring pattern that is provided on a semiconductor chip side of the flexible substrate, wherein a plurality of protrusions is formed on the wiring pattern during the step of forming the wiring pattern, by etching predetermined locations.
This makes it possible to form the gap preservation member during the step of forming the wiring pattern, thus enabling a shortening of the process.
The configuration could be such that through holes are provided in the flexible substrate at positions corresponding to the protrusions, and the external electrodes are provided through the through holes on a surface opposite to the surface of the flexible substrate on which the wiring pattern is provided.
An insulating resin could be painted onto positions of the protrusions acting as the gap preservation member corresponding at least to the semiconductor chip.
A semiconductor device that has been made by the above described method comprises:
a semiconductor chip having electrodes;
a flexible substrate disposed over and overlapping the semiconductor chip with a predetermined gap therebetween, the flexible substrate having external electrode formation portions within the overlapping region, the flexible substrate having connection portions electrically connected to the external electrode formation portions, the connection portions connected to the electrodes of the semiconductor chip; and
a gap preservation member provided at a position which excludes a position corresponding to the external electrode formation portions, the gap preservation member preserving the gap.
The active region of the semiconductor chip is preferably positioned within the overlapping region of the flexible substrate. This configuration makes it possible to add a function of protecting the active surface of the semiconductor chip without increasing the number of components, by causing the active region of the semiconductor chip to be protected by the flexible substrate.
In addition, a stress absorption layer could be positioned between the semiconductor chip and the flexible substrate.
Furthermore, the stress absorption layer could be provided in a position corresponding to the external electrode formation portion.
In particular, this gap preservation member could be formed by using at least part of the wiring pattern of the flexible substrate.
It is particularly preferable that a plurality of protrusions are provided in the wiring pattern, at least one of the plurality of protrusions forms the gap preservation member, and at least one of the rest of the protrusions forms a connection portion to an electrode of the semiconductor chip.
External electrodes could be formed on a side of the flexible substrate, the side of the flexible substrate being opposite to a side where the wiring pattern is formed, the external electrodes formed corresponding to a position where the at least one of the protrusions as the gap preservation member is formed.
The gap preservation member could be formed of resin and also function as the stress absorption layer. In such a case, the resin could be a thermoplastic resin.
It is possible to mount the above described semiconductor device on a circuit board. In other words, in a circuit board on which the above semiconductor device is mounted, any of the above described semiconductor device is electrically connected thereto by external electrode formation portions of the semiconductor device.
In addition, external electrodes that are formed on this external electrode formation portion could be connected directly to connecting portions of the circuit board.
A flexible substrate used in a semiconductor device is a flexible substrate comprising:
a base portion; and
a wiring pattern provided on one surface of the base portion, the wiring pattern having a plurality of protrusions formed integrally thereon, the base portion having through holes formed therein, each of the through holes corresponding to a position at which each of the plurality of protrusions are provided.
The plurality of protrusions preferably form connection portions to electrodes of the semiconductor chip and also a gap preservation member and an insulating layer is preferably provided on a surface of the protrusions corresponding to the gap preservation member.
Alternatively, the flexible substrate could comprise:
connection portions, the connection portions connected to electrodes of a semiconductor chip; and
a gap preservation member made of resin, the gap preservation member provided on a side where the connection portions are provided, the gap preservation member provided at a position that excludes the connection portions. The gap preservation member could be made from a thermoplastic resin.